Take three of the most used drones out there:

Ogre II optimal 1km, activation 4km, falloff 4km

Hammerhead II optimal 1km, activation 2km, falloff 3km

Warrior II optimal 1km, activation 1km, falloff 2km

And a sentry drone:

Curator II optimal 42km, activation 250km, falloff 24km

The activation range is the range at which the drone will start attacking the target it's been set on/goes for. It will then exit it's mwd cycle and start it's pew pew cycle (sentries don't move, hence the 250km activation range). It will orbit at optimal (I think, and the base optimal orbit at that, so 1km for combat drones at their orbit speed).

At some point (range) they will enter the mwd cycle again to get to the pew pew cycle again, but before that a higher optimal will get your drones better hits on moving targets (and for warrior IIs on non moving targets too, since they orbit at semi-lunie speeds).

Of course this pretty marginal for non sentry drones since even an optimal increase of 25% gives your drone an optimal of 1250m, which is almost certainly still less than their orbit range, so they will be shooting in falloff (actually not sure if the skill works for non combat drones, they generally have a pretty huge optimal though and will have the same activation as their tech I hull equivalents, so they will almost always be in optimal or mwd-ing).

For the sentry drone however, an increase of 25% would mean the curator has an optimal of well over 50km! Now we're talking, add an omnidirectional drone mod there and it gets another 20%.

So, it's useful.